1. Field of Invention
Bioremediation of BTEX, MTBE and other hydrocarbon and ether contaminants in subsurface soil.
2. The Prior Art
BTEX (benzene, toluene, ethylbenzene and xylenes) and MTBE (methyltertiarybutyl-ether) are gasoline components/additives known to be injurious to human health. They have been found to enter the water supply, for example, by leakage from underground gasoline storage tanks.
The use of “bionets” to destroy contaminants in ground water and subsurface rock and soil formations is known. A “bionet” is a subsurface zone containing microorganisms known to biodegrade the contaminants. A bionet is formed by inoculation of the microorganism into a well or subsurface zone or by creating an environment within a well or subsurface zone where such microorganisms, native to the site, will proliferate and effectively degrade the contaminant.
One prior art approach to formation of a bionet in a subsurface formation involves the use of hydraulic fracturing. See: W. J. Davis-Hoover, L. C. Murdoch, S. J. Vesper, H. R. Pahren, O. L. Sprockel. C. L. Chang, A. Hussain and W. A. Ritschel. “Hydraulic Fracturing to Improve Nutrient and Oxygen Delivery for In Situ Bioreclamation,” in: R. E. Hinchee and R. F Olfenbuttel (Eds.), In Situ Bioreclamation Applications and Investigations for Hydrocarbon and Contaminated Site Remediation, Butterworth-Heinemann, Stoneham, Mass., 1992, pp. 67-82.
The process of hydraulic fracturing at a well begins with the injection of fluid into the well, typically using a constant rate pump. The pressure of the fluid increases until it exceeds a critical value and a fracture is nucleated. A proppant is simultaneously pumped into the fracture as the fracture grows away from the well. Transport of proppant may be facilitated by using a viscous fluid, usually a gel formed from guar gum and water, to carry the proppant grains into the fracture. After pumping, the proppant holds the fracture open while the viscous gel breaks down into a thin fluid. The thinned gel is then pumped out of the fracture, leaving a layer of proppant grains in the subsurface. Hydraulic fracturing is one of the few techniques capable of placing substantial mases of solid compounds in the subsurface. Fractures containing hundreds of kilos of material have been created within a few meters of the ground surface, and much bigger fractures are certainly possible. Stacking flat-lying fractures offers the possibility of dissecting a contaminated site with closely spaced reservoirs of nutrients and oxygen.
In the prior art attempts to apply hydraulic fracturing in formation of bionets, oxygen has been the most important limiting factor. The introduction of oxygen into soil for bioremediation has traditionally been based on pumping oxygenated water or air into the soil. Both of these methods have significant limitations. For example, oxygen has a very limited solubility in water (about 8 PPM) and the lower the temperature of the water, the lower the solubility. This low oxygen carrying capacity of water means that great volumes of water need to be constantly added to the soil. This creates problems for spreading the contaminants around at a site and potentially into neighboring soil or into subsurface water. Also, this process requires extensive surface pumps and meters which are prone to failure and freezing.
Pumping air into soil creates problems because the air flow is resisted in the soil by the water capillarity pressures. These tend to be the locations where the contaminants are trapped and thus the oxygen never gets to the microorganisms near the contaminant. Air introduction also requires extensive surface equipment susceptible to failure and continuous maintenance.
In the aforementioned paper by W. L. Davis-Hoover et al, the authors reported an attempt to overcome the above-described problems of oxygen introduction by use of a solid oxygen source (SOS) in laboratory simulation of hydraulic fracturing. More specifically, they report the results of tests using sodium percarbonate encapsulated with ethylcellulose. However, it was found that the encapsulated percarbonate was exhausted after 24 hours.
Subsequently, Vesper et al reported the results of tests designed to evaluate the effectiveness of encapsulated sodium percarbonate as a source of oxygen to support biodegradation, again under laboratory test conditions. More specifically, the SOS used in these reported experiments consisted of sodium percarbonate microencapsulated with polyvinylidene chloride (PVDC). This SOS was used to support gram-negative bacteria (Pseudomonas) in biodegradation of propylene glycol (PPG) serving as a surrogate contaminant. See Vesper, S. J., L. C. Murdoch, S. Hayes, and W. J. Davis-Hoover, 1994 “Solid Oxygen Source for Bioremediation in Subsurface Soils”. J. Hazardous Materials, 36:265-274. However, the sodium percarbonate encapsulated with PVDC was found to become exhausted of its capability to supply oxygen in only a few weeks, thus suggesting only limited usefulness.